Communications System with Symmetrical Interfaces and Associated Methods

ABSTRACT

A communications system includes a physical layer device (PLD) and a logical link device (LLD), each having respective send and receive interfaces being substantially identical to define symmetrical interfaces for the system. Accordingly, design and manufacturing is simplified compared to conventional systems. In addition, advantages are also provided in terms of loopback capability and packaging options. The PLD comprises a PLD send interface including PLD parallel information outputs, and a PLD receive interface including PLD parallel information inputs. Similarly, the LLD comprises an LLD receive interface including LLD parallel information inputs, and an LLD send interface including LLD parallel information outputs. Parallel communications channels connect the PLD information outputs to respective LLD information inputs, and connect the LLD information outputs to respective PLD information inputs. The PLD send interface and the LLD send interface are substantially identical, and the PLD receive interface and the LLD receive interface are substantially identical to thereby define the symmetrical interfaces for the system. In view of the symmetrical interfaces, the PLD and the LLD may operate in a push-push configuration. Deskewing features are also provided.

RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.10/899,078, filed Jul. 27, 2004, which is a continuation of U.S. patentapplication Ser. No. 09/459,439, filed Dec. 13, 1999, which claims thebenefit of U.S. Provisional Application Ser. No. 60/112,379, filed Dec.14, 1998, each incorporated by reference herein”.

FIELD OF THE INVENTION

The invention relates to communications systems and methods, and, moreparticularly, to digital communications systems and associated methodsover parallel communications channels.

BACKGROUND OF THE INVENTION

Digital communications are widely used for the transmission of voice,data and video information. Such transmission can extend over largegeographical distances, between components within a personal computer,or only between adjacent circuit portions on an integrated circuit.Certain such communications applications benefit from or require theconversion of serial data into parallel data for simultaneoustransmission over parallel communications channels, or more generically,from M′ary symbols to N′ary symbols. At the receiving end, the paralleldata is desirably converted back into the serial data, and with the bitsor symbols in the correct order to avoid data errors.

Unfortunately, the demand for greater data transmission volumes and atever higher speeds, may result in skew at the receiver. In other words,the parallel communications channels may introduce different delays tothe parallel symbol strings they carry. Because of skew, the parallelsymbol strings at the receiver can then no longer be simply reassembledinto the starting data.

The skew problem with parallel communications channels has beenaddressed in a number of ways. For example, U.S. Pat. No. 4,677,618 toHaas et al. recognized the dispersion introduced by wavelength divisionmultiplexed communications channels over optical fiber. This patentdiscloses determining the relative delays between the channels basedupon detecting two bits in a given byte of data. The relative times ofarrival of the remaining bits in a byte are predetermined using therelative delay between the two detected bits and the knownfrequency-related dispersion characteristics of the transmission medium.Certain bits in each received byte may then be delayed using clock delaylines or registers, thereby accounting for skew.

Along similar lines, U.S. Pat. No. 5,157,530 to Loeb et al. alsodetermines and accounts for skew imparted by dispersion in fiber opticwavelength division multiplexing. Relative delays are used to controladjustable delay devices in each channel.

U.S. Pat. No. 5,408,473 to Hutchinson et al. is directed to a techniquefor synchronizing runlength-limited data transmitted over parallelcommunications channels. Block boundary synchronization is establishedduring connection initialization by using a property of a required HALTcode to detect block boundaries received in each channel. Skewcompensation is effected by comparing the times of detection of theblock boundaries in the two channels, and appropriately controlling avariable delay in at least one of the channels. If there is a subsequentloss of synchronization, detected transmission errors will eventuallyresult in connection reinitialization and reestablishment ofsynchronization. Unfortunately, the transmission of the fixed HALT codeto detect boundaries may result in false boundary detection. Moreover,since synchronization is not continuously maintained, the technique maybe impractical for higher data rates.

U.S. Pat. No. 5,793,770 to St. John et al. is directed to ahigh-performance parallel interface (HIPPI) to a synchronous opticalnetwork (SONET) gateway, and wherein electronic logic circuitry formatsdata and overhead signals into a data frame for transmission over afiber optic channel. Stripe skew adjustment is based upon SON ETframing, and, as such, the circuitry is relatively complicated,comprising as many as 20,000 logic gates, for example.

The difficulty with skew caused by parallel communications channels isalso an important issue to be addressed in communications channelsbetween integrated circuit devices. For example, higher transmissionspeeds increase the sensitivity to skew, as there is a smaller timewindow to correctly identify a received bit and have it properly alignwith bits received on the other parallel communications channels. Toprovide a higher aggregate transmission rate, the number of parallelcommunications channels can be increased, without increasing the speedof any given communications channel. However, this may result insignificant costs for the additional communications channels. Moreover,for communications between integrated circuits, increasing the number ofcommunications channels increases the number of pins needed forconnecting the IC. The number of pins and additional packagingcomplexity may significantly increase the costs of such approaches.

For communications channels between physical layer devices (PLDs) or PHYdevices, and logical link devices (LLDs), typical interfaces areasymmetrical and the devices are operated in a push-pull configuration.Because of the asymmetry, relatively expensive memory is required on thePLD since it is polled by the LLD, such as an asynchronous transfer mode(ATM) device. Further developments and improvements in thecommunications interface between a PLD and LLD are also hampered by theskew difficulty described above as a result of higher bit rates overlimited parallel communications channels.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of thepresent invention to provide a communications system and associatedmethods with simplified and efficient interfaces between a PLD and anLLD.

These and other objects, features and advantages in accordance with thepresent invention are provided by A communications system comprising aPLD and an LLD, each having respective send and receive interfaces beingsubstantially identical to define symmetrical interfaces for thecommunications system. Accordingly, design and manufacturing issimplified compared to conventional systems. In addition, advantages arealso provided in terms of loopback capability and packaging options. Inparticular, a channel loopback can be initiated in the PLD. Also, theLLD can be provided in two integrated circuit packages to ease pinoutrequirements.

The PLD comprises a PLD send interface including PLD parallelinformation outputs, and a PLD receive interface including PLD parallelinformation inputs. Similarly, the LLD comprises an LLD receiveinterface including LLD parallel information inputs, and an LLD sendinterface including LLD parallel information outputs. Parallelcommunications channels connect the PLD information outputs torespective LLD information inputs, and connect the LLD informationoutputs to respective PLD information inputs. The PLD send interface andthe LLD send interface are substantially identical, and the PLD receiveinterface and the LLD receive interface are substantially identical tothereby define the symmetrical interfaces for the communications system.In view of the symmetrical interfaces, the PLD and the LLD may operatein a push-push configuration.

The LLD may comprise, for example, an asynchronous transfer mode (ATM)device. The PLD may comprise one of a synchronous optical network(SONET) device or a synchronous digital hierarchy (SDH) device.

Yet another aspect of the invention is that the pin count of the PLD andLLD may be kept manageable by using higher speed parallel communicationschannels while accounting for skew. In particular, the PLD sendinterface may comprise a string-based framing coder for determining andappending a string-based framing code to each information symbol stringof information symbol strings to be transmitted in parallel overrespective first parallel communications channels, each string-basedframing code being based upon at least some of the information symbolsin the respective information symbol string. Also, the LLD receiveinterface may comprise a deskewer for aligning received parallelinformation symbol strings based upon the string-based framing codes.The information symbols may be binary bits, and the string-based framingcodes may be CRC codes, for example.

The deskewer may comprise a framer for framing information symbolstrings based upon the respective string-based framing codes, and analigner for aligning framed information symbol strings relative to oneanother and based upon the string-based framing codes. The aligner, inturn, may comprise at least one first-in-first-out (FIFO) deviceconnected to the framer for buffering framed information bit strings.The aligner may also include a FIFO controller for aligning framedinformation bit strings during at least one of a writing and a readingphase of the at least one FIFO device and based upon the string-basedframing codes.

The string-based coder and deskewing may also be provided for theinformation signals from the LLD to the PLD.

A method aspect of the invention is for making such a communicationssystem comprising the steps of: providing the PLD with a PLD sendinterface and a PLD receive interface, providing the LLD with an LLDreceive interface to be connected via parallel communications channelsto the PLD send interface, and further providing the LLD with an LLDsend interface to be connected to the PLD receive interface via parallelcommunications channels. Moreover, the PLD send interface and the LLDsend interface are preferably substantially identical, and the PLDreceive interface and the LLD receive interface are preferablysubstantially identical to thereby define symmetrical interfaces for thesystem.

In view of the symmetrical interfaces, the PLD and the LLD may operatein a push-push configuration. The LLD may comprise, for example, anasynchronous transfer mode (ATM) device. The PLD may comprise one of asynchronous optical network (SONET) device or a synchronous digitalhierarchy (SDH) device.

The method may further comprise the step of providing the PLD sendinterface with a string-based framing coder for determining andappending a stringbased framing code to each information symbol stringof information symbol strings to be transmitted in parallel overrespective parallel communications channels. Each string-based framingcode may be based upon at least some of the information symbols in therespective information symbol string. The method may also comprise thestep of providing the LLD receive interface with a deskewer for aligningreceived parallel information symbol strings based upon the string-basedframing codes.

Another aspect of the invention is directed to a method for operating acommunications system comprising a physical layer device (PLD) and alogical link layer device (LLD) connected by parallel communicationschannels. The PLD preferably includes a PLD send interface and a PLDreceive interface, and the LLD includes an LLD receive interfaceconnected to the PLD send interface. The LLD may also include an LLDsend interface connected to the PLD receive interface. The PLD sendinterface and the LLD send interface are preferably substantiallyidentical, as are the PLD receive interface and the LLD receiveinterface. The method of operating the system preferably comprises thestep of operating the PLD and the LLD in a push-push configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a first embodiment of acommunications system in accordance with the present invention;

FIG. 2 is a schematic bit position diagram from the output of the firstdevice as shown in FIG. 1;

FIG. 3 is a schematic bit position diagram from the input of the seconddevice as shown in FIG. 1 and illustrating skew;

FIG. 4 is a schematic bit position diagram from the FIFO device of thesecond device as shown in FIG. 1 and illustrating deskewing;

FIG. 5 is a schematic diagram of a framing state machine as may be usedin the second device as shown FIG. 1;

FIG. 6 is a schematic block diagram of an optical fiber embodiment of acommunications system in accordance with the present invention;

FIG. 7 is a schematic block diagram of a radio embodiment of acommunications system in accordance with the present invention;

FIG. 8 is a schematic block diagram of an infrared free space embodimentof a communications system in accordance with the present invention;

FIG. 9 is a schematic block diagram of another embodiment of acommunications system illustrating a bank of lower rate converterelectronics and incorporating the deskewing features in accordance withthe present invention;

FIG. 10 is a schematic block diagram of an optical fiber embodiment of acommunications system as shown in FIG. 9;

FIG. 11 is a schematic block diagram of a communications systemincluding a PLD and an LLD connected by parallel communications channelsin accordance with the present invention;

FIG. 12 is a more detailed schematic block diagram of the PLD sendinterface and LLD receive interface as shown in FIG. 11;

FIG. 13 is a schematic diagram of binning of the data and control bitsfor the PNG interface example in accordance with the present invention;and

FIG. 14 is a schematic block diagram of an example of a deskewingalgorithm in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout, and prime and multiple primenotation is used in alternate embodiments to refer to similar elements.

Referring initially to FIGS. 1-5, an embodiment of a communicationssystem 20 in accordance with the present invention is first described.The communications system 20 illustratively includes a first device 22and a second device 24 connected by parallel communications channels. Inthe illustrated embodiment, the communications channels are provided byelectrical conductors or wires 25-29, although other transmission mediamay be used to establish or define the parallel communications channelsas will be readily appreciated by those skilled in the art. Also in theillustrated embodiment four wires 25-28 are connected to carryinformation bits, while the fifth wire 29 carries a clock signal fromthe clock 42. The communications channel for the clock signal is notneeded in all embodiments, as the clock signal can typically berecovered if the received information bits have a sufficient number oftransitions as will also be appreciated by those skilled in the art.

As explained above in the Background of the Invention, parallelcommunications channels may present a skew problem especially where thebit rate is relatively high or the distance is relatively long. Forexample, for an 800 Mbs rate over electrical parallel conductors, skewmay limit separation distances to two inches or less.

For clarity of explanation, the following description will be withreference to transmitting binary information elements or information bitstrings. In other words, the term “information bit string” will be used,although those of skill in the art will understand that symbols otherthan binary one's and zero's can also be used in accordance with thepresent invention. For example, a three-level information symbol mayalso be used and benefit from the deskewing concepts described herein.

The first device 22 illustratively includes a string-based framing coder32 for determining and appending a string-based framing code to eachinformation bit string of information bit strings to be transmitted inparallel over respective parallel communications channels. “Appending”is meant to cover both prepending and postpending, although thoseskilled in the art will recognize that postpending may be preferred,since prepending may require more buffer memory.

A scrambler 34 is connected upstream from the string-based framing coder32. The scrambler 34 may be desirable to avoid long strings of nullvalues which could hinder clock recovery as will be appreciated by thoseskilled in the art. In other embodiments, the scrambler 34 may beconnected downstream from the string-based framing coder 32. Such anoptional scrambler may a self-synchronizing scrambler, such as a X̂43scrambler as will be appreciated by those skilled in the art.

An M'ary-to-N'ary mapper or converter 36 is illustratively connectedupstream of the scrambler 34. The M'ary-to-N'ary converter 36 convertsthe incoming Mbits to N parallel information bit strings for subsequenttransmission over the parallel communications channels. TheM'ary-to-N'ary converter 36 is conventional and requires no furtherdiscussion herein.

Returning again to the string-based framing coder 32, thisillustratively includes a string-based code generator 37 for generatingeach string-based framing code based upon at least some of theinformation bits in the respective information bit string. A multiplexer38 appends the string-based codes to the respective information bitstrings as will be appreciated by those skilled in the art. Anelectrical-to-medium converter 41 is connected between the output of themultiplexer 38 and the communications channels provided by the wires25-29. In this illustrated embodiment, the electrical-to-mediumconverter 41 may be provided by suitable electrical driver circuitry aswill be appreciated by those skilled in the art. In other embodiments,the electrical-to-medium converter 41 may connect to other transmissionmedia.

The second device 24 preferably includes a deskewer 45 for aligningreceived parallel information bit strings based upon the string-basedframing codes. The string-based framing codes and their use to deskewreceived information bit strings permit the information bits to betransmitted at high rates and/or over relatively long distances.

In one preferred embodiment, the string-based coder 32 comprises acyclic redundancy checking (CRC) coder for determining and appending CRCcodes to respective information bit strings. Thus, the deskewer maycomprise a CRC framer for framing the information bit strings based uponthe CRC codes. Of course, the second device 24 may also include an errordetection and correction circuit 47 using the CRC codes. Each CRC codemay be one of a CRC-4 to CRC-32 code, for example. For an informationbit string of 1024 bits, for example, a CRC-8 code may be sufficient toensure quick and accurate framing.

The string-based code may also include other bits in addition to thosespecifically based on the information bit string, such as the CRC codebits, for example. Some bits may be assigned as counting or identifyingbits to be used when the expected delay or skew was greater than asingle frame as will be appreciated by those skilled in the art. Ofcourse, other bits could be assigned for other purposes as well.

A particular advantage of the CRC coding is that a straightforwardhardware implementation can be achieved with a relatively small numberof logic gates as will be appreciated by those skilled in the art. CRCcodes are also resistant to false framing while adding relatively littleoverhead to the information bit strings. Fixed framing in contrast,would likely experience considerable false framing for a similar numberof code bits. If the number of fixed framing bits were increased toreduce false framing, the overhead may be considerable. Yet anotheradvantage of CRC codes is that they may also be used for error detectionand correction as they are conventionally used. Accordingly, the seconddevice 24 may optionally include the illustrated error detect andcorrection circuit 47, which needs no further discussion herein.

The deskewer 45 may comprise a framer 50 for framing information bitstrings based upon the respective string-based framing codes. Theillustrated deskewer 45 of the second device 24 also includes an aligner52 for aligning framed information bit strings relative to one anotherand based upon the string-based framing codes. The aligner 52 may, inturn, advantageously comprise at least one first-in-first-out (FIFO)device 53 connected to the framer 50 for buffering framed informationbit strings as shown in the illustrated embodiment. The aligner 52 alsoillustratively includes a FIFO controller 55 for aligning framedinformation bit strings during at least one of a writing and a readingphase of the at least one FIFO device and based upon the string-basedframing codes. The term “FIFO device” is used herein to include a FIFO,a shift register, and any other type of ordered storage element as willbe appreciated by those skilled in the art.

All of the information bit strings may have a same number of bits insome embodiments to simplify the system implementation. In otherembodiments, the bit strings could have different lengths as would beappreciated by those skilled in the art. Turning now to the front end ofthe second device 24, a sampler 56 is connected upstream from thedeskewer 45. The sampler 56 samples the received bit string based uponthe clocking pulses as will be appreciated by those skilled in the art.Ideally the sampler 56 samples the bit string at a bit midpoint. Theclock signal for the sampler 56 may come from the clock receiver 57 orfrom the recovered clock 58, the operation of both of which will beappreciated by those skilled in the art.

A medium-to-electrical converter 61 is connected between the sampler 56and the communications channels as provided by the wires 25-29. Ofcourse, other types of converters can be used for different transmissionmedia.

The deskewer 45 also illustratively includes a descrambler 46 fordescrambling the information bit strings, such as to facilitate clockrecovery at the second device 24. The descrambler 46 is illustrativelyconnected between the framer 50 and the FIFO device 53. In otherembodiments, the descrambler 46 may be connected downstream from theFIFO device 53 as will be appreciated by those skilled in the art. Ofcourse, in other embodiments, the descrambler 46 and the scrambler 34may not be needed at all.

Referring now more particularly to FIGS. 2-4, the deskewing inaccordance with the invention is described with reference to asimplified example. The table 62 in FIG. 2 illustrates the alignment ofsome of the information bits A-P and some of the CRC bits C11-C42. Thisis the proper alignment that would typically be produced at the outputof the first device 22 or at a relatively short distance therefrom aswill be appreciated by those skilled in the art.

As shown in the table 63 of FIG. 3, the second information bit stringfrom the top is out of alignment with the other information bit strings.Accordingly, the information bit string that would be recovered withoutdeskewing would be A, Z, C, . . . P. In other words, the information bitstring would be incorrect.

Now, as shown in the table 64 of FIG. 4, the deskewing of the presentinvention re-aligns the frames that may have been misaligned due toskew. Accordingly, the correct information bit string, A, B, . . . P, isproduced at the output. The communications system 20 and associateddeskewing method using the string-based framing codes advantageously andefficiently removes or accounts for the skew. This permits higher bitrates and/or longer transmission distances. The higher bit rates maypermit a reduction of pin count for communication between integratedcircuit chips. As the cost for additional pins and packaging complexitymay be relatively high, the present invention also permits lower costcommunications ICs having an aggregate communication rate that is stillrelatively high as will be appreciated by those skilled in the art. Aswill also be appreciated by those skilled in the art, although only onedirection of communication has been illustrated, other embodiments ofthe communications system 10 may include circuitry to implement areverse direction of communications. In other words, the presentinvention is also readily applicable to full-duplex communicationssystems as well. In addition, multiple receiving devices may beconnected to one or more transmitting devices as will also beappreciated by those skilled in the art.

Referring now more particularly to FIG. 5, some additional framing orsynchronization concepts are now explained. In particular, theillustrated state framing machine 70 has three states: a pre-sync state71, a hunt state 73 and a sync state 72. Transition between the hunt andpre-sync states is determined based upon a correct or incorrectstring-based framing code. The machine 70 changes from the pre-syncstate 71 to the sync state 72 if X consecutive correct codes aredetermined. The machine 70 transitions from the sync state 72 to thehunt state 73 if there are Y consecutive incorrect codes determined. Thestate framing machine 70 is very similar to state framing machines usedin other known data synchronizing applications as will be appreciated bythose skilled in the art.

The fundamental string-based framing coding and associated deskewingconcepts of the present invention have initially been explained withreference to parallel communications channels provided over wires 25-59.In other words, the communication system 20 operates over a wirelinetransmission medium. Another wireline application would includeoperation over a data bus, such as a PCI bus, for example. A typical PCIbus is limited to a relatively low frequency of about 60-70 MHZ.Accordingly, for greater information throughput wider buses are needed.The present invention overcomes this difficulty and can allow a PCI busto operate at a faster clock speed without additional bus width. Otherwireline transmission media include twisted copper pairs, and coaxialcables, for example, as will be appreciated by those skilled in the art.

Turning now additionally to FIGS. 6-8, several alternate embodimentswith respect to the transmission media are now described. For thecommunications system 20′ shown in FIG. 6, the communications medium isillustratively provided by two optical fibers 75. The first device 22′includes the string-based framing coder 32′, and the second device 24′includes the deskewer 45′. The communications system 20′ may alsoinclude other components as shown in FIG. 1 and described above.

A first wireless communications system embodiment 20″ is explained withparticular reference to FIG. 7. In this embodiment, radio transmittersand receivers 76, 77, respectively, and free space provide thecommunications channels over a wireless medium. The other components areindicated with double prime notation and are similar to those describedabove. FIG. 8 illustrates a second wireless communications system 20′″wherein infrared transmitters and infrared detectors 81, 82,respectively, provide the parallel communications channels over freespace as will be appreciated by those skilled in the art. Those of skillin the art will also appreciate other more specific implementations andapplications of the general communications systems 20, 20′, 20″ and 20′″described herein.

Other advantageous features and implementations of the present inventionare now described with additional reference to FIGS. 9 and 10. Moreparticularly, as shown in FIG. 9, the invention is also directed to acommunications system 120 including a first device 122 comprising aplurality of electrical-to-transmission medium converters 141, and asecond device 124 comprising a plurality of transmissionmedium-to-electrical converters 161. The electrical-to-transmissionmedium converters 141 are connected to respective ones of thetransmission medium-to-electrical converters 161 via at least onetransmission medium and defining parallel communications channelsbetween the first and second devices. In the illustrated embodiment, thetransmission medium is provided by the wireline 125. In otherembodiments, wireless and optical transmission media may be used. Aradio wireless medium is schematically indicated by antennas 113, 114,and an infrared or free space optical medium is indicated by source 115and detector 116.

The first device 122 comprises a string-based framing coder 132 fordetermining and appending a string-based framing code to eachinformation bit string of information bit strings to be transmitted inparallel over respective parallel communications channels, eachstring-based framing code being based upon at least some of theinformation bits in the respective information bit string as describedabove with respect to the string-based framing coder 32 as describedabove with reference to FIG. 1. The string-based coder 132 may include astring-based code generator and multiplexer as also described above withrespect to FIG. 1.

The second device 124 comprises a deskewer 145 for aligning receivedinformation bit strings based upon the string-based framing codes. Thedeskewer 145 may include the components and/or equivalents as describedabove for the deskewer 45 shown in the communications system 20 ofFIG. 1. Accordingly, lower rate converters 141, 161 can be used in thecommunication system 120 at significant costs savings and whileproviding a desired relatively high overall information throughput rate.The deskewing features account for any skewing that may occur throughthe parallel communications channels.

For longer distances, the first device 122 may further comprise amultiplexer 110 for multiplexing signals from the plurality ofelectrical-to-transmission medium converters 141 along a commontransmission medium, such as the wireline medium 125. In theseembodiments, the second device 124 also includes a demultiplexer 112connected to its transmission medium-to-electrical converters 161 aswill be appreciated by those skilled in the art. The medium multiplexingand demultiplexing may reduce the cost for the overall communicationssystem 120 for relatively large distances between the first and seconddevices 122, 124 as will also be appreciated by those skilled in theart.

For other embodiments, the multiplexer 110 and demultiplexer 112 may notbe needed. Those of skill in the art will readily be able to determinethe cost tradeoffs to implement the communications system 120 eitherwith or without the mux/demux.

One particularly advantageous use of the tradeoff in converter speedversus number of parallel communication channels is for opticalimplementations, such as for those in accordance with the synchronousoptical network (SONET) and/or synchronous digital hierarchy (SDH)standards. In particular, wavelength division multiplexing (WDM) anddense wavelength division multiplexing (DWDM) are highly developed andpermit a relatively large number of communications channels to beestablished at different optical wavelengths as will be appreciated bythose skilled in the art. In the illustrated communications system 120′,a larger number of lower rate OC-X converters 141′, 161′ may be used inplace of a lesser number of higher rate converters. The aggregate rateacross the fiber 175 can be relatively high, that is, N times the OC-Xrate. For example, an OC-192 converter may be 100 times the cost of aOC-48 converter. Accordingly, the communications system 120′ may be lessexpensive than comparable aggregate transmission rate systems usinghigher speed optical converters.

The different wavelengths multiplexed onto the fiber 175 travel atdifferent speeds through the fiber. In the past, these different speedshave been measured and a fixed offsets added to account for skew.Unfortunately, such approaches may not account of changes in skew as maybe caused by fiber aging and/or temperature cycling, for example. Thoseof skill in the art will appreciate the advantages in terms ofefficiency and simplicity provided by the string-based framing coder132′ and deskewer 145′ and associated methods in accordance with theinvention for the optical communications system 120′

Another communications system 200 in accordance with the invention isnow described with particular reference to FIGS. 11 and 12. Theillustrated communications system 200 includes a physical layer device(PLD) 201 and a logical link device (LLD) 202 connected thereto. The PLD201 includes a PLD send interface 203 which, in turn, includes PLDparallel information outputs 205-208 and at least one PLD control output211.

The LLD 202 comprises an LLD receive interface 204 which, in turn,includes LLD parallel information inputs 215-218 and at least one LLDcontrol input 221. The communications system 200 also includes firstparallel communications channels 225 connecting the PLD informationoutputs 205-208 to respective LLD information inputs 215-218. A secondcommunications channel 226 connects the illustrated single PLD controloutput 211 to the LLD control input 221 so that control signals are sentfrom the PLD to the LLD out-of-band from information signals.Accordingly, control speed is enhanced, and information throughputefficiency is not compromised. The number of first and secondcommunications channels 225 and 226 can be different in differentembodiments, as will be appreciated by those skilled in the art.

The LLD receive interface 204 further illustratively includes an LLDstatus output 222, and the PLD send interface 203 includes a PLO statusinput 212. A third communications channel 227 thus connects the LLDstatus output 222 to the PLD status input 212.

The PLO 201 also includes a PLD receive interface 230 including PLDparallel information inputs 231-234 and a PLD control input 235. The LLD202 further comprises an LLD send interface 240 including LLD parallelinformation outputs 241-244 and an LLD control output 245. In theillustrated embodiment, fourth parallel communications channels 250connect the LLD information outputs 241-244 to respective PLDinformation inputs 231-234. In addition, a fifth communications channel251 connects the LLD control output 245 with the PLD control input 235.

The PLD receive interface 230 may further include a PLD status output236, the LLD send interface 240 may further include an LLD status input246, and the communications system 200 may also include a sixthcommunications channel 252 connecting the PLD status output 236 to theLLD status input 246. In accordance with this aspect of the invention,the PLD 201 and LLD 202 are operable in a push-push configuration.

The push-push configuration overcomes a number of difficulties ofconventional push-pull configurations. The PLD 201 typically includeson-chip memory 260 which is relatively expensive. Although the LLD 202typically requires more memory, the memory 261 is less expensive sinceit is typically provided off-chip as will be appreciated by thoseskilled in the art. The push-push configuration relaxes the need forrelatively expensive on-chip memory 260 for the PLD 201, since the LLD202 does not throttle the PLD 201.

Another advantageous feature of the invention is that the interfaces203, 204, 230 and 240 may be made symmetric. In other words, the PLDsend interface 203 and the LLD send interface 240 may be substantiallyidentical, and the PLD receive interface 230 and the LLD receiveinterface 204 may be substantially identical to thereby definesymmetrical interfaces. Symmetrical interfaces may simplify design andmanufacturing and offer other advantages including loop-backcapabilities as indicated by the dotted loopback path 263 as illustratedbetween the PLD send interface 203 and the PLD receiver interface 230 aswill be appreciated by those skilled in the art.

The symmetry also permits the LLD 202 to be provided in two separateintegrated circuit packages functionally divided as indicated by thedashed horizontal line 264. Since the number of pins can be relativelylarge and account for a significant portion of the cost, the symmetryand ability to provide two separate ICs is a significant advantage ofthe present invention.

The LLD 202 may comprise an asynchronous transfer mode (ATM) device, forexample, as will be appreciated by those skilled in the art. Inaddition, the PLD may be a synchronous optical network (SONET) device ora synchronous digital hierarchy (SDN) device as will also be appreciatedby those skilled in the art. The LLD 202 may also be an SDL or HDLCdevice as will also be appreciated by those skilled in the art.

Yet another aspect of the communications system 200 is that the pincount of the PLD 201 and LLD 202 may be kept manageable by using higherspeed parallel communications channels while accounting for skew. Inparticular, as shown with particular reference to FIG. 11, the PLD sendinterface 203 may comprise a string-based framing coder 32″^(″) fordetermining and appending a string-based framing code to eachinformation bit string of information bit strings to be transmitted inparallel over respective first parallel communications channels, witheach string-based framing code being based upon at least some of theinformation bits in the respective information bit string. Also, the LLDreceive interface 204 may also comprise a deskewer 45″^(″) for aligningreceived parallel information bit strings based upon the string-basedframing codes.

The string-based framing codes may be CRC codes, for example. Theillustrated string-based coder 32″^(″) includes a string-based codegenerator 37″^(″) and multiplexer 38″^(″) as described above. Thescrambler may be added, but is not shown in the illustrated embodiment.An M'ary-to-N'ary converter 36″^(″) is upstream from the string-basedframing coder 32″^(″).

The deskewer 45″^(″) illustratively includes a framer 50″^(″) forframing information bit strings based upon the respective string-basedframing codes, and an aligner 52″^(″) for aligning framed informationbit strings relative to one another and based upon the string-basedframing codes. The aligner 52″^(″) includes least one first-in-first-out(FIFO) device 53″^(″) connected to the framer for buffering framedinformation bit strings; and a FIFO controller 55″^(″) for aligningframed information bit strings during at least one of a writing and areading phase of the at least one FIFO device and based upon thestring-based framing codes. A particular example of an efficient framingalgorithm is provided in the next portion of this description.

The coding and deskewing may be provided in the reverse direction oninformation and control signals from the PLD to the LLD. In someembodiments, the parallel communications channels may be provided overelectrical conductors as shown in the illustrated embodiment. Of course,other transmission media may also be used.

I. Example Interface

Having already described the general components, concepts, features andadvantages of the present invention, this description now includes aspecific example of an interface entitled the PNG interface. Of course,this example is meant to further describe the invention by way ofexample and should not be construed to be limiting of the invention.

PNG is an interface that allows the transport of data from one device toanother, at very high data rates, with a minimal number of pins. PNG issymmetrical, and push/push, and can be used at OC-48 rates extensible toOC-192 and beyond. In addition, PNG is designed to transport both ATMcells and packets concurrently.

As shown in FIG. 11 it can be see that PNG includes three kinds ofsignals: data, control, and status. Control is sent in the samedirection as data, and status is sent in the opposite direction. Thereare thus d+c+s bits that comprise an interface per direction. (Note thatunlike Utopia or Utopia like interfaces, PNG is symmetric, and is thuspush/push instead of push/pull.) Table 1 indicates representative orsuggested values for d, c and s, for various bit rates.

TABLE 1 Proposed Bit Widths and Rates OC-12 OC-48 OC-192 d 4 @ 155 Mb/s4 @ 622 MB/s 8 @ 1244 MB/s c 1 @ 155 MB/s 1 @ 622 MB/s 2 @ 1244 MB/s s 1@ 155 MB/s 1 @ 622 MB/s 2 @ 1244 MB/s

At 622 MB/s and above, controlling skew can be difficult. To ensurealignment between bits, and accomplish framing, each bit has its ownframing structure: for every n information bits transmitted, there is aCRC appended. The n information bits, along with the CRC may be referredto as an information frame. The suggested size of n is 1024 bits, andthe suggested CRC is a CRC-8 of polynomial x̂8+x̂2+x+1. However, this sizeand CRC can be varied dependent on an analysis to determine maximalprobabilistic acquisition times. Other suggested sizes are 512 bits witha CRC-4 or 2048 bits with a CRC-32. The CRC-4 requires the leasthardware, but may require considerable acquisition time because of highfalse framing probabilities. The CRC-32 requires more framing Circuitry.

In addition to the appended CRC, the data will also be scrambled using aframe synchronous scrambler of sequence length 127, of polynomialx̂7+x̂6+1. The scrambler shall be reset to 7'b111_(—)1111 on the mostsignificant bit of the byte following the CRC insertion. Note that thescrambling of information is performed to ensure a uniform distributionof 0's and 1's to allow for CDR.

It is proposed that a framing state machine similar to HEC delineationbe adopted as will be appreciated by those skilled in the art. Theframing state machine 70 is shown in FIG. 5, and has already beendescribed.

After frame is found, it is the receiver's responsibility to align theinformation across multiple bits. In the case of OC-48, the receiverwould frame on the 4 d bits and the 1 c bit, and after frame is acquiredby all five bits, data and control can be correctly extracted. Toaccelerate acquisition time, false-positive matches on the CRC can beignored if frame is not found on all five bits with a limited amount oftime (plus or minus a few bits, depending on the allowed skew betweenbits) In the case of OC-48 with 4 data bits and 1 control bit operationat 622 MB/s, the data and control would be binned as shown in FIG. 13,forming a relationship of 32 bits for every 8 control bits. This binningof course would be performed after information framing has occurred.

After binning of the control and data has occurred, the controlinformation is used to determine status of the associated data. Thisinformation includes Port ID (PID), Group ID (GID), Start of Packet (orcell), and End of Packet (or cell), which byte is the end of the packet.The information is given below in Table 2.

TABLE 2 Control and Data Fields 8 32 CONT PID DATA CONT GID STATUS DATA

The first bit of the control field indicates if the control octet (andits associated data field) is a continuation (i.e. part of) the previouscontrol octet. If CONT is 0, then this is the first octet of a newtransmission. Port ID (PID) is 7 bits wide, and is used to identify theport that the data is associated with. This results in up to 128 ports.If more ports are required, the Group ID (GID) field which is 3 bits, isused to extend the range of ports to 1024 (2̂10) The STATUS field is usedto convey the rest of the control information. The encoding of thestatus field is shown in Table 3.

TABLE 3 Encodings of STATUS 0000 NULL 0001 SOP 0010 CONT 0011 ABORT 0100reserved 0101 reserved 0110 reserved 0111 reserved 1000 EOP on previousA 1001 EPO on previous B 1010 EPO on previous C 1011 EPO on previous D1100 EOP on A 1101 EOP on B 1110 EOP on C 1111 EOP on D

Because the amount of control information that must be sent for eachchannel cannot be contained in one 8 bit octet, at least two 8 bitoctets must be sent before a port context switch can be performed.However, after this, a context switch can be performed on the next 4byte boundary (implying that the interface is 100% efficient formultiples of 4 byte packets, as long as the packet is 8 bytes or more)Note that any size packet, from one byte upwards, can be sent—less than8 bytes reduces the efficiency of the link.

Shown in Table 4 below is an example of null data being sent, followedby a 32 byte packet.

TABLE 4 Example of Null Data Followed by a 32 byte packet 0 PID x 1 GID0000 X 1 GID 0000 X 0 PID DATA 1 GID 0001 DATA 1 x 0010 DATA 1 x 0010DATA 1 x 0010 DATA 1 x 0010 DATA 1 x 1111 DATA

Note that with PID and GID, the number of channels that can be supportedin PNG is 1024. This could be extended to 2048 if necessary by reducingthe status size to 3 bits and extending the GID to 4 bits. However, thenthe data field would have to be utilized to convey information onpackets of sizes 1-4 bytes.

The status field is sent in the opposite direction of the data andcontrol, and is used to provide flow control on a per channel basis. Thestatus field is scrambled, and postpended with a CRC in the same fashionas the control and data. Once the status information has been extracted,it is interpreted according to the format shown below in Table 5.

TABLE 5 Status Information Format 7 3 2 4 PID GID R FSTATUS

As in the Control field, PID represents the Port ID, and GID representsthe Group ID, and these combined bits can be used to provide status onup to 1024 ports. R is reserved, and FSTATUS is used to convey thestatus of the FIFO associated with the port. The R field could be usedto grow either the GID or FSTATUS as necessary.

The encoding of the FSTATUS is shown in Table 6 below.

TABLE 6 Encodings of FSTATUS 0000 Fifo Empty 0001 . . . 0010 . . . 0011. . . 0100 . . . 0101 . . . 0110 . . . 0111 . . . 1000 . . . 1001 . . .1010 . . . 1011 . . . 1100 . . . 1101 . . . 1110 . . . 1111 Fifo Full

If only one bit status is required, the MSB bit of the FSTATUS can beemployed. The receiving side (defined as the side receiving data andcontrol but sending status) is responsible for determining the status ofits FIFOs, and conveying the information to the sending side (defined asthe side sending data and control but receiving status). A one bit fifostatus can be thought of as similar to TxClav in Utopia interfaces.

The sending side is required to maintain the status of every FIFO on thereceiving side. This status is continuously updated whenever a FIFOstatus changes at the receiver, and in the absence of any suchinformation to convey, the receiver round robins through all activeFIFOs to ensure that the sending side is continuously synchronized. Inthe case of a traditional PHY device as the sender, it does not requirestatus. As such, status information can be ignored by the PHY as asender.

A logical block diagram of the PNG interface is shown in FIG. 1 andneeds no further description.

II. Deskew Algorithm Example

Having described the general components, concepts, features andadvantages of the present invention, and an interface example thisdescription now includes a specific example of a deskew algorithm. Thisdeskew algorithm is meant to further describe the invention by way ofexample and should not be construed to be limiting of the invention.

As more bandwidth is needed in a network, customers desire the abilityto add bandwidth between two points in an on-demand basis. In aSONET/SDH network this bandwidth is normally provided by sending thenext larger concatenation level to the point needing the bandwidth.These networks have major limitations on the timeslot the newconcatenated signal can occupy. In accordance with the invention,bandwidth can be allocated/de-allocated at the STS-1/VC-4 level forSONET/SDH signals respectively with no restriction on the STS-1/VC-4signal used. At the receiving end, a mechanism must exist to align thesedisjoint streams into one stream without external intervention(software) and minimal hardware (gate count). This example addresses thesoftware intervention and gate count problem by implementing acompletely hardware based deskewing algorithm.

The SONET/SDH SPE deskewing algorithm allows the dynamic alignment ofmultiple STS-1/VC-4 signals within a user defined group. Accordingly,signals can be added or dropped dynamically from the user defined groupwithout introducing errors in the output signal.

As understood with reference to FIG. 14, the deskewing algorithm 279takes into account features of a SONET/SDH frame that allow a simplealgorithm to align SPEs of different STS-1/VC-4 signals. The algorithm279 includes four basic components: an input control logic block 280, awrite control logic block 281, data storage element block 285, and readcontrol logic block 286.

The input control logic block 280 contains the provisioned informationneeded to determine which STS-1/VC-4 signals are grouped together toform a Virtual group. This information is also used by other controlblocks for deskewing purposes.

The write control logic block 282 contains the write address generationlogic and the minimum write address and maximum write addresscalculation logic for each virtual group. A virtual group includes fromtwo (2) to forty-eight (48) STS-1 signals or two (2) to sixteen (16)VC-4 signals. The write address is synchronized to the J1I marker. Thisensures the J1 POH byte is always written into the same location in thedata storage memory. The write address always writes 783 (STS-1) or783*3 (VC-4) bytes between J1 markers. This is independent of incrementor decrement operations that have occurred to the incoming pointervalues as the signal passed through pointer processor elements in thenetwork. Simultaneously, during each row the maximum and minimum writeaddresses are calculated for all virtual groups. This information isused by the read control logic 286 in determining if an increment ordecrement operation is needed. This ensures all signals within the VCgroup stay aligned within the bounds of the FIFO depth.

The data storage element block 285 includes sufficient RAM to buffer onerow of data for each incoming STS-1 or VC-4 signal.

The read control logic block 286 ensures the read pointer moves/holds asthe phase relationship between the incoming signals change due topointer increments and decrements. An evaluation is made once per row todetermine it an increment operation (read address held during SPE time)or a decrement operation (advance read address during TOH time) isnecessary. The read pointer counts during SPE time only, except when adecrement operation is executed. If all incoming signals of the virtualgroup have the same J1 byte location the read and write pointers will beoffset by half the storage depth and the min/max values will be equalafter the de-skewing process.

In summary, the de-skewing algorithm allows any number of signals tomake-up a virtual group and keep these signals aligned as long as theincoming signal has a consistent number of bytes between markers and therelative offset between the markers are not greater than the FIFO depth.The description above is specific to SONET/SDH signals but thisinvention can be used for any type signals that must be aligned.

The main advantage of this approach is the simplicity of the deskewingalgorithm which takes into account the standard characteristics of aSONET/SDH frame structure and pointer movement behavior. The incomingsignals making up a virtual group are automatically aligned through thewrite algorithm and the read algorithm ensures the output is error freeas the incoming signals change phase with respect to each other. Thisinvention can be used in an alignment FIFO circuit to align VirtualConcatenated signals in a SONET/SDH network or perform high speeddeskewing in a packet network.

Indeed, many modifications and other embodiments of. the invention willcome to the mind of one skilled in the art having the benefit of theteachings presented in the foregoing descriptions and the associateddrawings. Therefore, it is to be understood that the invention is not tobe limited to the specific embodiments disclosed, and that modificationsand embodiments are intended to be included within the scope of theappended claims.

1. A communications system comprising: a physical layer device (PLD)comprising a PLD send interface including PLD parallel informationoutputs and at least one PLD control output, and a PLD receive interfaceincluding PLD parallel information inputs and at least one PLD controlinput; a logical link layer device (LLD) comprising an LLD receiveinterface including LLD parallel information inputs and at least one LLDcontrol input, and an LLD send interface including LLD parallelinformation outputs and at least one LLD control output; and parallelcommunications channels connecting said PLD information outputs torespective LLD information inputs, connecting said LLD informationoutputs to respective PLD information inputs, connecting said at leastone PLD control output to a respective at least one LLD control input,and connecting said at least one LLD control outputs to a respective atleast one PLD control input; said PLD send interface and said LLD sendinterface being substantially identical, and said PLD receive interfaceand said LLD receive interface being substantially identical therebypermitting said PLD and said LLD to operate in a push-pushconfiguration.
 2. A communications system according to claim 1 whereinsaid LLD comprises an asynchronous transfer mode (ATM) device.
 3. Acommunications system according to claim 1 wherein said PLD comprisesone of a synchronous optical network (SONET) device and a synchronousdigital hierarchy (SDH) device.
 4. A communications system according toclaim 1 wherein said PLD send interface comprises a string-based framingcoder for determining and appending a string-based framing code to eachinformation symbol string of information symbol strings to betransmitted in parallel over respective first parallel communicationschannels, each string-based framing code being based upon at least someof the information symbols in the respective information symbol string;and wherein said LLD receive interface comprises a deskewer for aligningreceived parallel information symbol strings based upon the string-basedframing codes.
 5. A communications system according to claim 4 whereineach information symbol comprises a binary bit; and wherein saidstring-based coder comprises a cyclic redundancy checking (CRC) coderfor determining and appending CRC codes to respective information bitstrings.
 6. A communications system according to claim 4 wherein saiddeskewer comprises a framer for framing the information symbol stringsbased upon the string-based framing codes.
 7. A communications systemaccording to claim 6 wherein said deskewer further comprises an alignerfor aligning framed information symbol strings relative to one anotherand based upon the string-based framing codes.
 8. A communicationssystem according to claim 7 wherein each information symbol comprises abinary bit; and wherein said aligner comprises: at least onefirst-in-first-out (FIFO) device connected to said framer for bufferingframed information bit strings; and a FIFO controller for aligningframed information bit strings during at least one of a writing and areading phase of said at least one FIFO device and based upon thestring-based framing codes.
 9. A communications system comprising: aphysical layer device (PLD) comprising a PLD send interface and a PLDreceive interface; a logical link layer device (LLD) comprising an LLDreceive interface and an LLD send interface; said PLD send interface andsaid LLD send interface being substantially identical, and said PLDreceive interface and said LLD receive interface being substantiallyidentical to thereby define symmetrical interfaces for thecommunications system.
 10. A communications system according to claim 9wherein said PLD and said LLD are operable in a push-push configuration.11. A communications system according to claim 9 wherein said LLDcomprises an asynchronous transfer mode (ATM) device.
 12. Acommunications system according to claim 9 wherein said PLD comprisesone of a synchronous optical network (SONET) device and a synchronousdigital hierarchy (SDH) device.
 13. A communications system according toclaim 9 wherein said PLD send interface comprises a string-based framingcoder for determining and appending a string-based framing code to eachinformation symbol string of information symbol strings to betransmitted in parallel, each string-based framing code being based uponat least some of the information symbols in the respective informationsymbol string; and wherein said LLD receive interface comprises adeskewer for aligning received parallel information symbol strings basedupon the string-based framing codes.
 14. A communications systemaccording to claim 13 wherein each information symbol comprises a binarybit; and wherein said string-based coder comprises a cyclic redundancychecking (CRC) coder for determining and appending CRC codes torespective information bit strings.
 15. A communications systemaccording to claim 13 wherein said deskewer comprises a framer forframing the information symbol strings based upon the string-basedframing codes.
 16. A communications system according to claim 15 whereinsaid deskewer further comprises an aligner for aligning framedinformation symbol strings relative to one another and based upon thestring-based framing codes.
 17. A communications system according toclaim 16 wherein each information symbol comprises a binary bit; andwherein said aligner comprises: at least one first-in-first-out (FIFO)device connected to said framer for buffering framed information bitstrings; and a FIFO controller for aligning framed information bitstrings during at least one of a writing and a reading phase of said atleast one FIFO device and based upon the string-based framing codes. 18.A communications system according to claim 12 wherein said firstparallel communications channels are provided over electricalconductors.
 19. A method for operating a communications systemcomprising a physical layer device (PLD) and a logical link layer device(LLD) connected by parallel communications channels, the PLD including aPLD send interface and a PLD receive interface, the LLD including an LLDreceive interface connected to the PLD send interface, and the LLD alsoincluding an LLD send interface connected to the PLD receive interface,the PLD send interface and the MJD send interface being substantiallyidentical, and the PLD receive interface and the LLD receive interfacebeing substantially identical, the method comprising the step of:operating the PLD and the LLD in a push-push configuration.
 20. A methodaccording to claim 19 wherein the LLD comprises an asynchronous transfermode (ATM) device.
 21. A method according to claim 19 wherein the PLDcomprises one of a synchronous optical network (SONET) device and asynchronous digital hierarchy (SDH) device.
 22. A method according toclaim 19 further comprising the step of providing the PLD send interfacewith a string-based framing coder for determining and appending astring-based framing code to each information symbol string ofinformation symbol strings to be transmitted in parallel over respectiveparallel communications channels, each string-based framing code beingbased upon at least some of the information symbols in the respectiveinformation symbol string; and further comprising the step of providingthe LLD receive interface with a deskewer for aligning received parallelinformation symbol strings based upon the string-based framing codes.23. A method according to claim 22 wherein each information symbolcomprises a binary bit; and wherein the string-based coder comprises acyclic redundancy checking (CRC) coder for determining and appending CRCcodes to respective information bit strings.
 24. A method according toclaim 22 wherein the deskewer comprises a framer for framing theinformation bit strings based upon the string-based framing codes.
 25. Amethod according to claim 24 wherein the deskewer further comprises analigner for aligning framed information bit strings relative to oneanother and based upon the string-based framing codes.
 26. A methodaccording to claim 25 wherein each information symbol comprises a binarybit; and wherein the aligner comprises: at least one first-in-first-out(FIFO) device connected to the framer for buffering framed informationbit strings; and a FIFO controller for aligning framed information bitstrings during at least one of a writing and a reading phase of the atleast one FIFO device and based upon the string-based framing codes. 27.A method according to claim 19 further comprising the step of providingthe LLD send interface with a string-based framing coder for determiningand appending a string-based framing code to each information symbolstring of information symbol strings to be transmitted in parallel overrespective parallel communications channels, each string-based framingcode being based upon at least some of the information symbols in therespective information symbol string; and further comprising the step ofproviding the PLD receive interface with a deskewer for aligningreceived parallel information symbol strings based upon the string-basedframing codes.
 28. A method according to claim 27 wherein eachinformation symbol comprises a binary bit; and wherein the string-basedcoder comprises a cyclic redundancy checking (CRC) coder for determiningand appending CRC codes to respective information bit strings.
 29. Amethod according to claim 27 wherein the deskewer comprises a framer forframing the information symbol strings based upon the string-basedframing codes.
 30. A method according to claim 27 wherein the deskewerfurther comprises an aligner for aligning framed information symbolstrings relative to one another and based upon the string-based framingcodes.
 31. A method according to claim 30 wherein each informationsymbol comprises a binary bit; and wherein the aligner comprises: atleast one first-in-first-out (FIFO) device connected to the framer forbuffering framed information bit strings; and a FIFO controller foraligning framed information bit strings during at least one of a writingand a reading phase of the at least one FIFO device and based upon thestring-based framing codes.